An initial tension is a force of a tension spring to be contracted before being deformed by an external force. As shown in FIG. 1, the tension spring starts to be deformed when a force corresponding to its initial tension is applied thereto. A length of the tension spring can be shortened by using the initial tension, so that a space can be effectively utilized and a size of a component can be minimized.
In order for the tension spring to have an initial tension, a coil spring has to be wound by a minus pitch.
However, the minus pitch has the following problems.
Since the minus pitch is invisible, it is difficult to implement a precise value and a large value.
Furthermore, in the conventional method using a remaining stress, the stress is released when the coil spring is wound in a heating process, or when the coil spring is thermally processed after being wound in a cooling process. Accordingly, it is very difficult to provide an initial tension to the coil spring.